| With the rapid development of industrialization and urbanization in China,more and more heavy metals are discharged into the environment.In the process of the whole society adhering to the road of sustainable development,the problem of heavy metal pollution in farmland soil has become a global key problem endangering the ecological environment and human health.Therefore,it is imperative to develop a practical and effective method to reduce the toxicity of soil heavy metals and repair the soil polluted b y complex heavy metals.At present,a large number of technologies have been used to repair the soil contaminated by heavy metals,but soil stabilization remediation is widely favored because of its simple operation,high efficiency and eco-friendly.Nanoscale zero-valent iron（nZVI）has a unique core-shell structure,excellent reaction activity and high adsorption capacity.However,due to the magnetism,van der Waals force and nano-size effect,nZVI is prone to oxidation and agglomeration after entering th e environment,which affects its adsorption capacity and activity.Palygorskite（PAL）,as a natural clay mineral,has rich mesoporous structures and a large number of negative charges on its surface,which makes it have good adsorption capacity and ion exc hange capacity.Loading nZVI on the surface of PAL can effectively prevent the agglomeration of nZVI and give full play to their remediation effect on heavy metals in soil.In this study,PAL-nZVI stabilization material was synthesized,screened and optimi zed by the liquid-phase reduction method.The tested soil was collected from the farmland around Silong Town,Baiyin City,Gansu Province.The contents of various heavy metals in this area exceeded the standard,espec ially the content of Cd,was more than 19 times higher than the second-level standard of Soil Environmental Quality Standard（GB15618-1995）.The surface morphology,minerals and functional groups of the new material were analyzed,and the stabilization mechanism was revealed combined with the experimental results.The effects of PAL-nZVI on soil physical and chemical properties,bioavailability and chemical speciation of heavy metals were clarified by stabilization test and pot test.The effects of PAL-nZVI on the growth of corn and the content of heavy metals in plants were investigated,and the effects of PAL-nZVI on the remediation and ecotoxicity of heavy metal contaminated soil were evaluated comprehensively.The results provide an important theoretical basis for the potential application value of PAL-nZVI in farmland soil polluted by complex heavy metals.The main results are as follows:（1）PAL-nZVI stabilizer can be prepared by the liquid-phase reduction method.nZVI is loaded on the surface of PAL successfully.PAL plays a good role in dispersing and supporting nZVI,and reduces the agglomeration of nZVI greatly.There are a large number of free and associated functional groups（such as-OH,Fe-O、≡Si-OH、Al-O、Si-Al）on the surface of PAL-nZVI,which is conducive to form ligands with heavy metal ions.Heavy metals can be stabilized through complexation,cation exchange or electrostatic interaction.PAL-nZVI has mild pH regulation ability.When the load ratio o f PAL to nZVI is 1:1 and 2:1,the soil pH value increases by more than 1 pH unit compared with CK.The soil cation exchange capacity and conductivity changed simultaneously.（2）When the loading ratio of PAL and nZVI was 2:1,PAL-nZVI had the best effect on reducing the bioavailability of six heavy metals.At this time,the average concentrations of Cu,Pb,Cd,Ni,Cr and Zn extracted from CaCl 2 decreased by33.64%,24.27%,57.64%,74.07%,66.12%and 23.02%respectively compar ed with the blank control group（CK）.Application of PAL-nZVI can better reduce the bioavailability of heavy metals in soil than the application of PAL or nZVI alone,indicating that there may be a synergistic effect between PAL and nZVI in the stabilization of heavy metals.After the application of PAL-nZVI,the heavy metals in soil changed from high active form to low active form.The residual Cd,Pb,Cu,Ni and Zn increased by 23.06%,38.56%,5.95%,24.07%and 18.21%respectively compared with CK.Althou gh the change of residual Cr was not obvious,the reducible Cr increased by 24.20%compared with CK,which realized the stabilization of heavy metals in soil.PAL-nZVI can also reduce the bioavailability and bioaccumulation of heavy metals in corn,to prom ote plant growth.The fresh weight of corn was 43.02±2.92 g/pot,which was 98.43%higher than CK,35.1%and 49%higher than that adding of PAL or nZVI alone.（3）The optimized PAL-nZVI was added to the polluted soil in different dosages.After a period of stabilization,the physical and chemical properties of the soil changed significantly.After 30 days of incubation,soil pH,EC,CEC,available nutrients,organic matter and available iron tended to be stable.At this time,when the dosage of PAL-nZVI was 8%,the soil pH increased by 1.77 pH units compared with CK.When the dosage of PAL-nZVI was 6%-8%,EC decreased by about 40%and CEC increased by more than 20%compared with CK.The contents of soil available nitrogen and phosphorus decreased by 12.85mg/kg and 3.14mg/kg respectively,and the content of soil organic matter decreased by 11.67%compared with CK.The content of available potassium increased by 6.58mg/kg compared with CK.The addition of PAL-nZVI had a certain effect on soil available Fe.At the beginning of incubation,the content of available Fe in soil increased rapidly with the increase of PAL-nZVI dosage.Compared with CK,the content of available Fe in soil increased by 98.10%（8%dosage）at 15 days.With the extension of incubation time,Fe（Ⅱ）/Fe（Ⅲ）in soil was re-fixed by oxidation precipitation,so that the content of available Fe in soil decreased continuously.At 45 days of incubation,the content of available Fe in soil increased by 33.48%com pared with CK.The dynamic monitoring of available Fe in soil shows that the addition of PAL-nZVI will not cause soil iron toxicity.（4）The CaCl2 extracted contents and the leaching toxicity of heavy metals in soil decreased significantly with the incre ase of PAL-nZVI`dosage.30 days is the key period for the stability of heavy metals,at which time the heavy metals in soil tend to be stable.The fraction of acid-soluble and reducible decreased significantly,while the fraction of oxidizable and residue with low bioavailability and ecotoxicity increased significantly.When the dosage of PAL-nZVI was 6%and the incubation time was 30 days,the percentage of residues Cu,Pb,Cd,Ni,Cr and Zn in soil increased by 8.65%,33.75%,27.24%,26.47%,9.54%and 33.26%respectively compared with CK.The comprehensive potential ecological index（RI）also decreases with the increase of the PAL-nZVI dose.When the dosage of PAL-nZVI exceeds 6%and the incubation time exceeds 30 days,the RI can decrease by more than 30%compared with CK.In most cases,when the dosage of PAL-nZVI is 6%and 8%,there is no significant difference in the stabilization effect of heavy metals.Considering the cost and soil secondary pollution,the optimal dosage of PAL-nZVI is determined to be 6%.（5）PAL-nZVI can significantly promote the growth of corn.When the addition of PAL-nZVI was 6%,the fresh weight of corn increased by 189.99%compared with CK.With the increase of PAL-nZVI dosage,the enrichment of heavy metals in various parts of corn showed a downward trend.Compared with CK,the contents of Cd,Pb,Cu,Ni,Cr and Zn decreased by 54.91%,47.11%,56.47%,42.11%,82.05%and 50.15%in shoot part,r espectively.The contents of Cd,Pb,Cu,Ni,Cr and Zn in the root part decreased by 44.10%,33.12%,48.47%,50.60%,33.80%and 53.44%respectively.The enrichment of heavy metals in the shoot part decreased more significantly than that of the root part.There was a significant positive correlation between the bioavailability of heavy metals in soil and the metal content in various parts of corn.Therefore,PAL-nZVI,as a cheap,efficient and environment-friendly new environmental protection material,is a feasible and potential soil heavy metal stabilizer.It can be used to improve the farmland soil contaminated by composite heavy metals and has a certain application value. |
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